Positional marker systems for optical tracking systems and, in particular, medical tracking systems, are known in principle. For example, a reflector or reflective referencing system is known from DE 19 639 615 C2 (U.S. Pat. No. 6,351,659 B1), wherein the positional markers are formed as passive markers and include spheres with a reflective covering or coating. Optical radiation reflected off of these spheres can be detected by a camera-assisted tracking system, and their spatial position can be determined. Instruments, patients or treatment apparatus coupled to the spheres can be spatially detected and tracked, thereby providing a surgeon visual assistance during surgery.
Further, active markers are known that function in a similar manner as passive markers, but are powered by power sources such as batteries or power packs. Active markers are light emitting devices, such devices being known, for example, from U.S. Pat. No. 5,197,476. U.S. Pat. No. 5,383,454 and U.S. Pat. No. 5,831,735 also show examples of non-contact, optical tracking with the aid of positional markers.
A disadvantage of previously known, active markers such as, for example, LED markers, is the relatively large dimensions of the markers and/or the active or emitting portion of the markers. More specifically, the markers take up a relatively large space and are difficult to attach to smaller or very thin objects. Also, LEDs provide a very non-homogeneous light pattern.
As for the passive markers, the problem arises that they have to be relatively large in order to be detected by the cameras. Relatively large markers, however, can become partially covered or tarnished while being used, or, due to manufacturing tolerances, can exhibit a shape which is not entirely regular. In the case of such passive markers, their center, which precisely reproduces the location of the marker, can be ascertained by calculating a “focus”. The focus can be determined by analyzing a projected external shape of the marker and calculating a center point. If the markers are partially covered, tarnished or of different sizes, however, it may not be possible to precisely calculate the center point. Errors arise, which, in applications that demand a high degree of accuracy (e.g., surgical applications) are undesirable. Also, if a marker becomes tarnished or covered, for example, recording the marker from two different cameras (e.g., two cameras having different perspective views of the marker) may produce recordings having different shapes. The same applies to active markers (e.g., LEDs) since they have non-homogeneous emissions.